The invention is directed toward the field of free-space optical communications, and more particularly, to a method and apparatus for automatically aligning the transmitter and receiver in a free-space optical communication system.
In wireless optical communication systems, the optical signal propagates in free space. In contrast to radio frequency (RF) communication systems, optical wireless communication systems are extremely directional. Thus, precise alignment is required between the transmitting unit and the receiving unit. The highly directional nature of wireless optical communication systems, however, provides the advantage of improved security, since the optical signal can only be intercepted along the path of the transmitted light. Another advantage of the optical wireless link is that the optical portion of the spectrum is not regulated by the government. Thus, a government license is not required to operate the transmitter and receiver, unlike a comparable radio frequency wireless communication system. More importantly, the bandwidth or information carrying capacity of optical wireless systems is much greater than that of RF wireless communication systems.
Fiber-based communication systems require the use of an optical fiber. Wireless optical communication systems have an advantage over such fiber-based communication systems in that the wireless communication systems do not require a physical connection between the transmitter and the receiver. In an urban environment, such as New York City, it can be difficult to install a physical connection between two buildings, especially if the buildings are separated by a street or another building. A wireless optical link only requires an unobstructed path between the transmitter and the receiver, which is often easier to achieve in an urban environment than a physical link. Wireless optical communication systems are particularly suitable for use where temporary high capacity data links between two installations are required, such as in an emergency relief operation for a disaster area or in military operations.
Wireless optical systems include a transmitting unit, for forming a transmitted beam, aimed at a receiving unit that collects the received beam. Typically, the optical signal to be transmitted is emitted from a semiconductor laser. The emitting facet of the laser (or an optical fiber into which the laser is coupled) lies at the front focal plane of the transmitting unit. The received signal is typically collected on a photodetector (or an optical fiber connected to the photodetector) positioned at the rear focal plane of the receiving unit.
As previously indicated, optical signals are extremely directional. Thus, the transmitting unit and the receiving unit must be precisely aligned with one another. Nonetheless, atmospheric diffraction effects can cause the transmitted beam to deviate from the carefully aimed path (beam wander). In addition, the alignment can be degraded as a result of temperature variations or movement of the transmitting unit or the receiving unit, for example, when the structure upon which the transmitting unit or the receiving unit is mounted moves.
Automatic beam tracking techniques have been used to compensate for alignment degradation and to ensure alignment of the transmitting unit and the receiving unit. Conventional automatic tracking techniques typically utilize a beacon signal that is generated by a separate laser using a different wavelength than the primary information-carrying signal. The beacon signal, which is aligned with the main beam, travels along the optical path and is redirected to dedicated alignment hardware, including a video camera. The dedicated alignment hardware determines whether the beacon signal (and thus, the primary information-carrying signal) is out of alignment and determines an appropriate alignment correction, if necessary, in a well-known manner. In addition to the added expense from such dedicated hardware, the beacon signal may exhibit different transmission properties, since the beacon signal is transmitted at a different wavelength than the primary information-carrying signal.
A need therefore exists for an automatic tracking technique that aligns the transmitting unit and the receiving unit using the primary information-carrying optical signal itself. A further need exists for an automatic tracking technique that aligns the transmitting unit and the receiving unit using the same wavelength as the primary information-carrying signal.
Generally, a method and apparatus are disclosed for aligning the transmitting unit and the receiving unit in an optical wireless communication system. The receiving unit includes an objective optic, such as a lens, and an optical bundle positioned at the focal point of the lens. According to one aspect of the present invention, the optical bundle is comprised of an array of optical fibers, arranged surrounding the receiving fiber. The receiving unit also includes a number of detectors that measure the optical signal strength on a corresponding fiber in the optical bundle. The array of fibers is used to detect the location of the received signal relative to the receiving optical fiber and to provide feedback to adjust the orientation of the optical bundle to optimize the received signal strength.
An alignment process utilizes the optical signal strengths measured by the surrounding fibers in the array to detect the location of the received signal relative to the receiving optical fiber, and to provide feedback to adjust the orientation of the receiving unit to optimize the received signal strength. Each outer optical fiber is connected to a corresponding optical signal detector that generates error signals that are proportional to the degree of misalignment between the receive signal and the receiving fiber. When misalignment occurs between the received signal and the receiving fiber, some of the incident received signal will be captured by one or more of the outer optical fibers. The amplitude of each of the generated signals are then compared to each other, thereby giving a direction in which to drive the optical bundle back into alignment with the received signal.
The present invention provides automatic tracking using the information-carrying optical signal, without the need for a separate laser. In one embodiment, the fibers in the array and the receiving fiber terminate in the same plane. In further variations, the receiving fiber is recessed relative to the surrounding fibers to prevent the optical signal from terminating in the cladding of the receiving fiber as the receiving unit initially loses alignment. The receiving fiber can be recessed relative to the surrounding fibers, for example, by appending an extension bundle or a silica disk to the optical bundle to add additional length to each fiber in the array.
A more complete understanding of the present invention, as well as further features and advantages of the present invention, will be obtained by reference to the following detailed description and drawings.